Sewage wastewater having a concentration of waste solids is typically collected in a wastewater container such as a septic tank. In the wastewater collection container the solids distribute into an upper horizontal scum layer and a lower horizontal sludge layer with a relatively clear horizontal liquid layer therebetween. Liquid from the relatively clear horizontal layer is intermittently discharged from the container as effluent, while ideally, all the solids remain in the collection container to be decomposed by the action of anaerobic bacteria. However, some solids are nevertheless discharged from a conventionally operated wastewater container because, as the decomposition of solids proceeds, gas is produced. Bubbles of product gas attach to some solids, which causes those solids to migrate from the sludge layer to the scum layer; the gas-lifted solids return to the sludge layer when the attached gas bubbles are released. These migrating solids may become entrained in the effluent discharged from the container, consequently degrading the quality of the effluent, and increasing the extent, and thus the expense, to which the effluent requires secondary treatment.
Various filtering devices have been associated with the wastewater container outlet in an attempt to prevent the discharge of solids with the liquid effluent. For example, a cylindrical mesh screen may surround a gravity flow outlet, a siphon intake or the intake of a discharge pump. Cylindrical mesh screens are shown, for example, in Graves U.S. Pat. No. 5,207,896, Grimshaw U.S. Pat. No. 3,662,890 and Ball U.S. Pat. No. 4,439,323. However, such a screen tends to become clogged with adhering solids which may even cause the screen to collapse. The clogged or collapsed screen must then be removed from the container and cleaned or replaced. As shown in Zabel U.S. Pat. No. 4,710,295, it is also known to cause the effluent to flow through the open bottom of a filtering device containing in its walls horizontally oriented elongate slots out through which the effluent flows to a gravity flow outlet of the wastewater collection container. However, such slots are still subject to clogging and they also permit horizontally oriented solids to enter the effluent flow.
Thus, a need exists for a more effective method of filtering solids from the effluent from a wastewater collection container while overcoming the problems of the prior art systems.
According to one aspect of the present invention such a need is satisfied by immersing a mesh screen filter in wastewater having a concentration of waste solids which have been allowed to distribute into a scum layer and a sludge layer with a horizontal layer of liquid therebetween, surrounding the filter with a housing, and exposing the interior surface of the housing to the horizontal layer of liquid through a plurality of apertures in the housing. Liquid from the horizontal layer flows through the apertures into the housing and thereafter the liquid flows from one side of the filter to an opposite side thereof through a mesh screen filtering surface area greater than the interior surface area of the housing. Thereafter, the liquid flows through a liquid effluent outlet of the container. This filter is very efficient and consequently provides a high quality effluent, while resisting clogging and resultant collapse of the mesh screen surface area.
According to another aspect of the invention, a filter having multiple filtering surfaces forming multiple enclosures is immersed in the wastewater and surrounded by the housing. The interior surface of the housing is exposed to the horizontal layer of liquid through a plurality of apertures in the housing. Liquid from the horizontal layer flows through the apertures in the housing and thereafter the liquid flows from one side of the filtering surface to an opposite side thereof in parallel through the multiple filtering surfaces. The multiple filtering enclosures are particularly resistant to clogging and thus to collapse, and require cleaning only as often as the container requires pumping to remove the accumulated sludge.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.